How to make a Dithering effect in blender?
I assume you are referring to this kind of dithering:


  Dither is an intentionally applied form of noise used to randomize quantization error, preventing large-scale patterns such as color banding in images. Dither is routinely used in processing of both digital audio and digital video data, and is often one of the last stages of audio production to compact disc.


Basically, it's used to break up bands that appear when you have too few colors in your image. For example, the image on the left is without dithering, the image on the right is with dithering:

 



You can turn up the amount of dithering noise in Render settings > Post processing, however this is really only meant for removing banding artifacts which sometimes occur in certain lighting setups (without reducing the number of colors).

For example (click on image for full size):





Note that blender can output full floating point deep color EXR images, so unless you plan on reducing the number of colors later (in which case you would want to dither the image when you reduce the colors), I'm not sure that more dithering would really do anything more than what the render settings option already does (breaking up subtle banding artifacts).